multimeasure rests

[Stefan Thomas]

Dear community,
because I'm awfully bad in mathematics I wrote a function for calculating the correct length of multimeasure rests.
Although it doesn't cause a crash (which is a huge sucess for me as a "programmer"), it doesn't work as expected. Instead of showing one goupt of multimeasure rests, it shows every single multimeasure rest.
Does someone have an idea to avoid the problem?
Here is my snippet:
 %%%%%%%%%%%%%%%%%
\version "2.14.2"
xR = #(define-music-function (parser location  ANFANG  LAST x ) (number? number? ly:music?   )
    #{ %
    \repeat unfold $(+ 1(- LAST ANFANG))  $x
#}) 
\layout { \context { \Score skipBars = ##t } }
\new Staff {
   R1*5 % this works as expected
   \xR #6 #11 R1 % this, unfortunately, not, it shows  6 single barrests
  c'1
}